A considerable amount of the worldwide imaging paper base is resin coated. The maximum speed at which a polymeric coating can be applied to an imaging paper base is often limited by the bond strength between the paper and the polymer. As speed increases and or temperature decreases, the strength of the bond between the polymer and the paper tends to decrease. This is a key consideration in the manufacture of photographic paper supports, since chemicals used in the aqueous photographic processing will tend to penetrate into the support between the polymer and the paper if the bond is poor. This will leave unsightly marks around the edges of the paper after processing.
It is, therefore, necessary to compromise between a high-speed production process and a high quality photographic product. One way to overcome this conflict is to increase the temperature of the polymer. This method is appropriate as long as the temperature is not so high that decomposition of the polymer results in deleterious physical properties or photoactive substances, which will fog the emulsion. Griggs (U.S. Pat. No. 3,582,337) claims polymer extrusion temperatures from 304.degree. C. to 343.degree. C. to be used at speeds of between 61 and 305 m/min. The temperatures that Griggs teaches are adequate to assure reasonable bond, but thermal degradation in the polyolefin results in occasional product imperfections (as mentioned in U.S. Pat. No.5,503,968), which are not tolerable by today's discerning customers. These imperfections have since been reduced by the addition of antioxidants such as 4,4'-butylidene-bis(6-tert-butyl-meta-cresol). These antioxidants are adequate for reducing spot imperfections, however they also degrade bond considerably. Thus, it is no longer possible to run at the speeds claimed by Griggs and still achieve good bond at these temperatures.
Another way to overcome poor bond is to use corona discharge treatment as described in U.S. Pat. No. 3,411,908. This technique is applied to the paper base before laminating. The corona discharge technique tends to "activate" the surface resulting in better bond once the polymer is applied. Another technique that has been used is the application of flame as described in U.S. Pat. No. 5,147,678. This approach utilizes the flame caused by the burning of natural gas, which impinges on the paper support. Again, this technique activates the paper, giving it better bond after the polymer is applied. One possible disadvantage of this technique is the possibility that flame treatment dries out the paper. Since moisture is necessary to facilitate the curing of the hardener in the photographic emulsion, this reduced moisture may diminish productivity in the sensitizing operation.
Honma (U.S. Pat. No. 4,481,289) describes the use of ozone, which can be applied to the molten polymer. This method activates the polymer instead of the paper support again increasing the bond after the polymer is laminated onto the paper. In this application, Honma claims a maximum polymer extrusion temperature of 300.degree. C. A maximum speed of 183 m/min is demonstrated which Lee (U.S. Pat. No. 5,503,968) points out is rather slow in today's environment. Lee describes a synergistic effect when flame is used in conjunction with ozone and demonstrates that speeds of greater than 400 m/min are possible. Unfortunately, as described above, this may have the disadvantage of drying the paper.
When imaging supports are subject to variations in ambient conditions over long periods of time the image-containing and resin layers tend to deteriorate into a mass of cracks which are aesthetically undesirable and which, in extreme cases, extend over the entire print completely destroying the image. All polymers are inherently prone to chemical degradation that leads to loss of mechanical properties. They undergo thermal degradation during processing such as extrusion of thin films, and photo-oxidative degradation with long-term exposure to light. TiO.sub.2 catalyzes and accelerates both thermal and photo-oxidative degradation. In the art of resin coating imaging papers, the melt polymers are extruded at high temperatures and are also subjected to high shear forces. These conditions may degrade the polymer, resulting in discoloration and charring, formation of polymer slugs or "gels", and formation of lines and streaks in the extruded film from degraded material deposits on die surfaces. Also, thermally degraded polymer is less robust than non-degraded polymer for long-term stability, and may thereby shorten the life of the print.
Hindered phenol antioxidants are commonly used alone or in combination with secondary antioxidants to stabilize polymers during melt processing, but provide little protection from long term photo-oxidation. The phenolic type anti-oxidants also decrease the ability of the resin to adhere to the paper during the high speed extrusion process. They are also responsible for some forms of oxidative atmospheric gas yellowing in prints stored in the dark. This undesirable color may develop on the print or around the print edge with archival keeping, and has been attributed to colored oxidation products of phenolic antioxidants that are formed in the dark in the presence of white pigments such as TiO.sub.2.
In U.S. Pat. No. 4,582,785 it is suggested using a polymeric hindered amines with greater than 2500 molecular weight as the sole stabilizer, when added to polyethylene coated photographic paper, can improve their photostability. In this patent a polymeric hindered amine is claimed as the sole stabilizer for both thermal processing and light stability in a single layer of a polymeric material, polyethylene. The adhesion of the polymeric hindered amine-containing resin to the paper is poor.
Furthermore it is desirable to make imaging elements that use other polymers than polyethylene or in combination with polyethylene. The use of such materials as polyester and or polypropylene may add improved stability and durability to the element as well as improved gloss and sheen.
There remains a need to provide an imaging support that contains pigments that are extrusion processable at low temperatures and high speeds without drying the paper, creating gels, or creating photoactive products which will fog the photographic emulsion. In addition, it must have exceptional long-term resistance to degradation and embrittlement when exposed to light and other environmental stresses, while providing an imaging support that has exceptional dark stability and prevents discoloration during dark keeping. These properties are most desirable and have significant commercial value.